1. Field of the Invention
The present invention relates to an inverter cooker, and more particularly to an inverter cooker having a high voltage/low voltage separating device for electrically separating a low voltage circuit portion from a high voltage circuit portion, which can protect a user from mishap caused by a high voltage leaking to the low voltage circuit portion, such as a function key input device, and which prevents also the low voltage circuit portion from malfunctioning.
2. Description of the Prior Art
FIG. 1 is a block diagram of a conventional inverter cooker. According to FIG. 1, a conventional inverter cooker is provided with a first rectifier 100 for rectifying an input AC source; a smoothing filter 101, composed of a choke coil L1 and a capacitor C1, for smoothing a ripple voltage rectified by the first rectifier 100; an inverter section 102 which is composed of a working coil 102a which heats food in a container 112 with an eddy current generated by an electromagnetic induction effect obtained by switching the direct current outputted from the smoothing filter 101, a resonance capacitor C2 which constitutes a series resonance circuit with the working coil 102a, a switching device Q1 and a damping diode D1; an inverter controller 103, composed of a voltage detector 103a, a triangular wave generator 103b, and a comparator 103c, for detecting a voltage from the inverter section 102 and generating a pulse width modulation (hereinafter referred to as PWM) signal according to the detected voltage; and a switching controller 104 which controls the the on/off operation of the switching device Q1 in the inverter section 102 according to the PWM signal generated from the inverter controller 103.
The inverter cooker is also provided with a current detector 105 for detecting a current from the input AC source; a step-down transformer 108 for lowering the voltage of the input AC source; a second rectifier 109 for recifying the voltage lowered by the step-down transformer 108; a function key input device 110 for entering cooking function key signals, for example `high-level cooking`, `medium cooking`, `low-level cooking`, `keeping warm` and so forth; a system controller 111 for controlling the operation of the inverter cooker according to the function key signal from the function key input device 110; a negative feedback amplifier 106 for comparing the current detected by the current detector 105 with the control signal outputted from the system controller 111; and an integrator 107 for integrating the compared value outputted from the negative feedback amplifier 106, and outputting the integrated value to the comparator 103C in the inverter controller 103.
In the conventional inverter cooker constructed as above, commercial AC source is rectified to a ripple voltage by the first rectifier 100, and then smoothed to a real DC by the smoothing filter 101. Since the AC source is connected with the step-down transformer 108, the AC voltage is lowered by the step-down transformer 108. The lowered AC voltage is rectified by the second rectifier 109 and then supplied as a low voltage VCC to the function key input device 110 and the system controller 111. After the DC voltage output from the smoothing filter 101 enters the inverter section 102, this voltage is switched by the switching device Q1 and becomes continuing resonance state by the working coil 102a and the resonance capacitor C2. Accordingly, a large resonance current flows through the working coil 102a and the eddy current is induced to the container 112 by the electromagnetic induction effect and thereafter food in the container will be heated. The switching device Q1 is driven by the PWM signal controlled by the triangular wave generator 103b and the comparator 103c, the damping diode D1 absorbs counter-electromotive voltage caused by the working coil 102a when the switching device Q1 goes off and thus plays a role in protecting the switching device Q1.
The voltage detector 103a in the inverter controller 103 detects the resonance voltage generated by the working coil 102a, and outputs the detected resonance voltage to the triangular wave generator 103b. The triangular wave generator 103b generates a triangular wave signal, which decides the on/off timing of the switching device Q1 in the inverter section 102, according to the detected voltage of the voltage detector 103a and outputs the triangular wave signal to the negative input terminal of the comparator 103c.
Meanwhile, the current detector 105 detects the current from the input AC source as a voltage value and outputs the detected value to the negative input terminal of the negative feedback amplifier 106. The negative feedback amplifier 106 compares the input voltage with the reference voltage outputted from the system controller 111, and outputs the compared value to the integrator 107. The integrator 107 integrates the compared value in order to determine an on/off timing of the switching device Q1 in the inverter section 102, and outputs the integrated value to the positive input terminal of the comparator 103c through a diode D2.
The comparator 103c compares the triangular wave signal from the triangular wave generator 103b with the integrated signal from the integrator 107, and outputs the PWM signal for determining an on/off timing of the switching device Q1 in the inverter section 102, to the switching controller 104. Thus, the switching device Q1 is turned on/off according to the PWM signal, causing the working coil 102a also to be switch-control-led.
However, the conventional inverter cooker has the disadvantages that, since the high voltage (about 800 V) line is directly connected with the second rectifier 109, the inverter controller 103, the function key input device 110, and the system controller 111 which work with the low supply voltage, the user likely to be harmed by the electrical shock and the like, when the user operates the function key in situations where that the function key input device is infiltrated by humidity or any other alien substance. Furthermore, in these systems, the system controller 111 is apt to malfunction.